[SelectionDAG] Make ISD::isConstantSplatVector always return an element sized APInt.

[SelectionDAG] Make ISD::isConstantSplatVector always return an element sized APInt.

This partially reverts r311429 in favor of making ISD::isConstantSplatVector do something not confusing. Turns out the only other user of it was also having to deal with the weird property of it returning a smaller size.

So rather than continue to deal with this quirk everywhere, just make the interface do something sane.

Differential Revision: https://reviews.llvm.org/D37039

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311510 91177308-0d34-0410-b5e6-96231b3b80d8